Adhesion of fluoropolymer topcoat

ABSTRACT

A METHOD FOR IMPROVING THE ADHESION OF A FLUOROCARBON POLYMER COATING TO ITS SUBSTRATE WITH A COMPOSITION OF 10-75% OF LITHIUM POLYSILICATE, 25-90% OF A FLUOROCARBON POLYMER AND A LIQUID CARRIER.

United States Patent Oflice 3,697,309 Patented Oct. 10, 1972 3,697,309 ADHESION F FLUOROPOLYMER TOPCOAT Ervin R. Werner, Jr., Levittown, Pa., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del. No Drawing. Continuation-in-part of application Ser. No. 45,945, June 12, 1970, which is a division of abandoned application Ser. No. 778,828, Nov. 25, 1968. This application Dec. 22, 1970, Ser. No. 100,764

Int. Cl. B44d 1/36; B32b 15/08 US. Cl. 11772 14 Claims ABSTRACT or m DISCLOSURE A method for improving the adhesion of a fluorocarbon polymer coating to its substrate with a composition of -75% of lithium polysilicate, 25-90% of a fluorocarbon polymer and a liquid carrier.

CROSS-REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of application Ser. No. 45,945, filed June 12, 1970, which in turn is a divisional of application Ser. No. 778,828, filed Nov. 25, 1968, both now abandoned.

BACKGROUND OF THE INVENTION SUMMARY OF THE INVENTION I have found that adhesion of a fluorocarbon polymer coating to its substrate can be significantly improved if a composition of (a) particulate fluorocarbon polymer, (b) lithium polysilicate, and (c) a liquid carrier is used to prime the substrate before the fluorocarbon polymer topcoat is applied.

The lithium polysilicate I use in my compositions is an aqueous lithium polysilicate containing about through about 35%, by weight, of silica as SiO It has a SiO /Li O mol ratio of about 4-25/1 and is free of alkali metals, other than lithium, to the extent that only those alkali metals which cannot be removed by ion-exchange are present.

The lithium polysilicate preferred for use in my compositons contains about through about by weight, of silica as Si0 and has a SiO /Li O mol ratio of 4-10/1.

Especially preferred is the lithium polysilicate which contains about 20% through about 25%, .by weight, of silica as SiO and has a SiO /Li O mol ratio of 45.5 1.

The lithium polysilicate even more preferred contains about 20% through about 25%, by weight, of silica as SiO and has a SiO /Li O mol ratio of 8-9/1.

Methods for preparing the aqueous lithium polysilicates suitable for use in my compositions are described in US. Pat. No. 3,455,709. The portions of that patent which describe preparation of lithium polysilicates are incorporated into this application by reference.

The lithium polysilicate is ordinarily present in my compositions at a concentration of about 10% through about by weight of the total of polysilicate and fluorocarbon polymer solids, preferably about 15 through about 40%. The especially preferred compositions contain about 25% of polysilicate.

The fluorocarbon polymers I use in my compositions are those of hydrocarbon monomers, preferably ethylenically unsaturated, completely substituted with fluorine atoms or a combination of fluorine atoms and chlorine atoms. Included in this group are polytetrafluoroethylene, copolymers of tetrafluoroethylene and hexafluoropropylene in all monomer unit ratios, and fluorochlorocarbon polymers such as polymonochlorotrifluoroethylene.

The fluorocarbon polymer I use is particulate. The particles should be small enough to pass through the nozzle of a spray gun without clogging it and small enough to give the resulting film integrity. In ordinary situations, the particle should be no larger than about 0.35 micron in the longest dimension.

Although I can use a dry flour or powder of fluorocarbon polymer and provide a liquid carrier separately, I prefer to use the polymer in the form of an aqueous dispersion because it is most easily obtained on the market in that form. Dispersions of fluorocarbon polymer in Organic liquids miscible with water, such as ethanol, isopropanol, acetone and cellosolves, can also be used. In either case, the liquid also serves as a carrier for my composition.

The fluorocarbon polymer is ordinarily present in my compositions at a concentration of about 25% through about 90%, by weight of the total of polysilicate and fluorocarbon polymer solids, preferably about 60% through about The especially preferred compositions contain about 75% of fluorocarbon polymer.

My compositions are made by mixing proper amounts of an aqueous lithium polysilicate and a fluorocarbon polymer dispersion. The compositions can be pigmented by preparing a suitable pigment dispersion by conventional techniques and then simply adding this pigment dispersion to the composition. The total solids content of these compositions can be 35-45% by weight.

The resulting primer composition is applied in the usual way. If the surface to be coated is metal, it is preferably pretreated by grit-blasting, by the flame spraying of metals or metal oxides, by frit-coating the substrate, or by a combination of these methods.

After application, the primer coat is air-dried and then baked at 230-400 C. for 10 to 20 minutes.

This primer coat is then topcoated with a conventional clear or pigmented fluorocarbon polymer enamel, and baked in the usual way, to give an adherent fluoropolymer coating.

Although I get the best coatings with the two-coat system just described, I also get strongly adherent fluorocarbon polymer coatings with a one-coat operation. To do this, I apply a thicker coat of the primer composition which may contain such pigments and surfactants as are necessary; this coat is then dried and baked as in the twocoat system. The advantage of this one-coat system in terms of labor cost is obvious.

My compositions are most useful for priming metal cookware, especially frypans, for coating with polytetrachutes, conveyors, dies, tools such as saws, files and drills,

hoppers, and other industrial containers and molds.

DESCRIPTION OF THE PREFERRED EMBODIMENTS- Those skilled in this art will be able to practice my invention more easily by first referring to the following illustrative examples. In these examples, all parts are by weight.

EXAMPLE 1 A primer composition was prepared by thoroughly mixing Pigment dispersion 30 E. I. du Pont de Nemours and Company.

The pigment dispersion was made by mixing 7 Parts Water 69.4 Sodium polynaphthylene sulfonate 0.5 Red iron oxide 5.3 Carbon black 0.7

and then ball-milling the mixture for 16 hours.

An aluminum frypan was wiped with acetone to remove grease and then grit blasted. The pan was then cleaned of grit particles with a blast of compressed air and the primer composition sprayed on the panto a thickness of 0.2-0.4 mil (dry).

The pan was then baked for 20 minutes at 230 C. after which it was topcoated with an enamel having the composition Parts Polytetrafluoroethylene dispersion, aqueous, 50%

solids Sodium salt of the sulfuric acid ester of a mixture of long chain alcohols (predominantly lauryl alcohol) by spraying the enamel over the primer to a thickness of 0.6-0.8 mil (dry) and then baking this at 400 C. for 20 minutes.

I The resulting coating of polytetrafluoroethylene adhered well to the frypan, with no blistering or peeling after extended use in the kitchen.

EXAMPLE 2 A primer composition was prepared by thoroughly mixing Parts Polytetrafluoroethylene dispersion, aqueous, particle size about 0.2 micron average (longest dimension) 60% solids 375 Lithium polysilicate 85 375 Pigment dispersion 65 Sodium lauryl sulfonate 7.5

The pigment dispersion was made as, shown in Ex- 4 this primer composition and topcoated, as shown in Example 1, to give a coating of polytetrafluoroethylenewhich adhered well to the frypan, with not blistering or peeling after extended use.

EXAMPLE 3 A primer composition was prepared by thoroughly mixing Parts Polytetrafluoroethylene dispersion, aqueous, particle size about 0.2 micron average (longest dimension), 60% solids 477 Lithium polysilicate .477 Pigment dispersion 83 The pigment dispersion was made by mixing Parts Water 67 Sodium polynaphthylene sulfonate 3 Aluminum silicate 10 Carbon black 20 and then pebble-milling the mixture for 16 hours.

An aluminum fry-pan was grit-blasted and frit-coated. The primer composition was sprayed on the pan to. a thick ness of 0.2-0.4 mil (dry). The pan was baked for 20 minutes at 230 C. and then topcoated, as shown in Example 1, togive a coating of polytetrafiuoroethylene which ad- 'hered well to the frypan, with no blistering or peeling after extended use.

I claim:

1. A method for improving the adhesion of a fluorocarbon polymer coating to a substrate, the method comprising (a) applying to the substrate a film of a composition consisting essentially of (1) about 10% through about 75%, by weight of the total of (1) and (2) solids, of lithium polysilicate containing about 15 through about 35%, by weight, of silica as $102, having an SiO /Li O mol ratio of 4-25/ 1, and being free of alkali metal ions other than lithium;

(2) about 25 through about by weight of the total of (1) and (2) solids, of a particulate polymer of ethylenically unsaturated hydrocarbon monomers completely substituted with fluorine atoms or a combination of fluorine'atoms and chlorine atoms; and

(3) a liquid carrier;

(b) curing the film in (a);

(c) applying to the film in (b) a film of a fluorocarbon polymer topcoat composition; and then ((1) curing the topcoat composition of (c).

2. The method of claim 1 wherein the composition contains about 15% through about 40% of lithium polysilicate and about 60% through about 85% of a fluorocarbon polymer.

3. The method of claim 2 wherein the lithium polysilicate in the composition has a 'SiO /Li O mol ratio of 4-10/1.

4. The method of claim 2 wherein the lithium polysilicate in the composition has a SiO /Li 0 mol ratio of '8-9/ 1.

5. The method of claim 2 wherein the fluorocarbon polymer in (a) (2) is polytetrafluoroethylene.

6. The method of claim 1 wherein the composition contains about 25% of lithium polysilicate and about 75% of a fluorocarbon polymer.

7. The method of claim 6 wherein the lithium polysilicate in the composition has a SiO /Li Omo1 ratio of 4-10/ 1.

8. The method of claim .6 wherein the lithium polysilicate in the composition has a SiO /Li O mol ratio of 8-9/ 1.

9. The method of claim 8 wherein the fluorocarbon polymer in (a) (2) is polytetrafluoroethylene and the liquid carrier in (a) (3) is Water.

10. The method of claim 6 wherein the fluorocarbon polymer in (a) (2) is polytetrafluoroethylene.

11. The method of claim 1 wherein the lithium polysilicate in the composition has a SiO /Li O mol ratio of 4-10/ 1.

12. The method of claim 1 wherein the lithium polysilicate in the composition has a SiO /Li O mol ratio of 8-9/1.

13. The method of claim 1 wherein the fluorocarbon polymer in (a) (2) is polytetrafiuoroethylene.

14. The method of claim 1 wherein the liquid carrier in (a) (3) is water.

6 References Cited UNITED STATES PATENTS 2,710,266 6/1955 Hochberg 117-132 CF X 2,825,664 3/1958 Huntsberger 117--232 3,455,709 7 1969 Sears 106-84 2,825,706 3/.1958 Sanders 117-75 X 3,047,421 7/1962 Taylor 117-161 UF X FOREIGN PATENTS 587,496 11/1959 Canada 260-29.6 F

RALPH HUSAOK, Primary Examiner US. Cl. X.R. 

